Without the ingress of corrosive species into concrete during its service life, or contamination of the mix ingredients by corrosive species, reinforcing steel tends to passivate in concrete. This passivation effect, brought about by the alkaline concrete pore solution, results in negligible corrosion rates. A Pourbaix diagram illustrates passivity of iron under alkaline conditions - the concrete pore solution pH is usually reported to range from 12-14.

One mechanism by which the passive surface conditions of reinforcing steel can be disrupted, and serious corrosion damage can set in, is known as carbonation. Through the ingress of carbon dioxide from the external atmosphere and its reaction with the pore solution, the pH of the pore solution tends to decrease (become more acidic). A decrease in internal pH levels to around 8 has been reported (4). The carbonation "front" gradually penetrating into concrete can be revealed on cross sections treated with an indicator solution. When this "front" reaches the embedded reinforcing steel, it is clearly more vulnerable to corrosion damage. Relatively porous concrete, wet/dry exposure cycles, low concrete cover and relatively high levels of carbon dioxide have been associated with an increased corrosion risk.

A more complex reinforcing steel corrosion mechanism is chloride induced attack. Harmful chloride species can originate from the original mix ingredients or from the external environment, notable de-icing salts applied in cold climates and marine exposures. The complex nature of chloride ingress into previously uncontaminated concrete and electrochemical interaction with the reinforcing steel can be appreciated when factors such as free chlorides, chemically bound chlorides, microscopic and macroscopic pore structures and crack paths, diffusion and capillary suction transport mechanisms, and "competition" between hydroxide and chloride anions in combining with iron cations are considered. The multitude of variables involved makes it difficult to define a specific threshold chloride level for avoiding corrosion damage (expressed as a weight percentage of cement).

Once conditions conducive to corrosion damage on the reinforcing steel surface have been established, the formation of voluminous corrosion products will generate tensile stresses in the concrete and subsequent spalling on the concrete cover. Extensive cracking in, and loss of, the protective cover obviously leave the reinforcing steel particularly vulnerable to further corrosion damage from the external environment. Therefore, a major challenge exists in obtaining early warning of internal corrosion damage, before serious and costly damage is visually apparent on the outside of existing structures.

PAGE 1 2 3 next